Illumination of high-power LEDs (HP LEDs) is typically accomplished by means of a constant current source for at least two reasons. First, light produced by an LED is directly related to current flowing through it, i.e., the number of electrons reaching an LED's junction is proportional to the number recombining to produce light. Second, LEDs have exponentially related forward current and voltage (I-V) characteristics in which small changes of forward voltage produce large variations in current. Thus, controlling for voltage is more challenging than regulating current, particularly as the I-V characteristics vary in response to heat generated by the LED.
Voltage regulators are readily configurable to produce a constant current suitable for powering LEDs. Two types of voltages regulators include switch-mode and linear types of regulators. Switch-mode voltage regulators rapidly switch a transistor to control energy in inductive elements based on the duty cycle of the switching. Switch-mode circuits are fairly efficient but they are more complex than linear type voltage regulators to implement and cause undesirable ripple current effects due to rapid switching. Linear voltage regulators, including so-called LDO types of linear voltage regulators, vary resistance of a regulation device, e.g., a pass transistor, in accordance with a load to produce a constant output voltage. The regulation device, therefore, is made to act like a variable resistor—continuously adjusting a voltage divider network to maintain a constant output voltage and continually dissipating the difference between the input and regulated voltages as waste heat. Because these voltage regulators tightly regulate the voltage drop between an output node and a reference voltage node (e.g., ground), a fixed resistor connected between these two nodes yields a source of constant current. This remains true for both high-side and low-side current source configurations.
All linear voltage regulators expect an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount, called the dropout voltage, is the input-to-output differential voltage at which a circuit ceases to regulate against further reductions in input voltage. This point typically occurs when the input voltage approaches the output voltage. For example, if the dropout voltage of a regulator is 1 V and the desired operating output voltage of the regulator is 5 V, then the input voltage should be maintained at 6 V or higher. If the input voltage drops below this value, then the output voltage will fail to reach 5 V. Dropout voltages vary depending on the load on the regulator—usually increasing under higher load—due to the internal resistance of the pass transistor and associated circuitry.
LDO voltage regulators, as the name implies, are a specific class of direct current (DC) linear regulator capable of regulating output voltage even when the supply voltage is somewhat close in value to the output voltage. LDO voltage regulators are characterized by a pass transistor maintained in a region of operation—i.e., saturation and linear/triode regions for, respectively, for BJTs and MOSFETs. In this region, the transistor behaves as a low-value resistance, thereby creating a dropout voltage. Additional voltage is lost by circuitry used to configure the LDO voltage regulator as a source of constant current.